Cost-optimization device for utility-scale photovoltaic power plants

ABSTRACT

This invention is embodied in a cost-optimization device for the layout and construction of a utility-scale photovoltaic (“PV”) power plant. The optimization device employs a set of algorithms designed to find the most cost-effective solution under given conditions. The algorithms are written in computer machine readable code and are highly customizable for the specific tracker equipment requirements and owner/builder/maintainer specifications or preferences. 
     The preferred optimization device comprises three principal stages (or “units”) of computing: (1) an objective-state unit, (2) an optimum-feasible unit, and (3) a grading unit. In the first stage, the objective-state unit cost-optimizes site grading by orienting a ruling line between a maximum and a minimum pile reveal length for each tracker in the project (the objective-state solution”). When compared to the existing site topography, the ruling line indicates cost-optimized cut and fill locations. 
     In the second stage, the optimum-feasible unit modifies the objective-state solution to satisfy given geometric constraints of the project. More specifically, the optimum-feasible unit employs a project-wide mesh (“system mesh”) to check whether the objective-state solution complies with the project&#39;s geometric restrictions. If not, the optimum-feasible unit applies a marching algorithm to incrementally adjust each ruling line until it finds a solution that minimizes site grading while complying with the project&#39;s geometric restrictions (the “optimum-feasible solution”). 
     In the third stage, the grading unit modifies the optimum-feasible solution to satisfy non-geometric constraints of the project (the “final-state solution”).

FIELD OF THE INVENTION

The present invention relates to cost-optimizing site grading for utility-scale photovoltaic power plants.

BACKGROUND

Utility-scale solar photovoltaic power plants require mounting structures to which the solar modules are attached to. The mounting structures can be either fixed-tilt type or solar tracking type. In the second case, the tracking structures are mechanisms providing the solar modules with varying orientation to maximize the energy yield of the solar plant.

Among the different types of solar tracking structures, the most commonly used in the industry are the so called ‘single axis trackers’. See, FIG. 1. These consist of a horizontal rotating axis mounting a multiplicity of modules. The north-south orientation and slow rotation of the horizontal axis adjusts the orientation of the solar modules as a function of the sun's position, so as to minimize the incident angle of the solar beams onto their surface. The horizontal rotating axis is supported by means of a series of vertical steel piles, transmitting the structural loads to the ground. The rotating axis and the vertical piles supporting it are mechanically connected by means of bearings on top of each pile.

Horizontal single-axis trackers inherently require small assembly tolerances to ensure the adequate alignment of its moving parts, which is achieved through the corresponding alignment for the bearings at the top of the piles.

If the terrain topography is not perfectly flat, the length of the piles can be designed to absorb terrain irregularities under each tracker, so to maintain the required alignment of the axis or axle. Alternatively, the terrain can be graded to smooth the topographic irregularities. It is straightforward that there is a trade-off between the terrain grading intensity and the length of the piles, therefore there is an optimum design point to be found which minimizes the overall cost of grading plus steel piles. In addition, there are project-specific geometric restrictions such as (i) minimum pile height to ensure sufficient gap between the modules and the ground when the tracker is deployed, (ii) maximum pile height to facilitate mechanical assembly by hand, (iii) maximum angular deviation from the horizontal line the tracker rotating axis is set to, (iv) maximum elevation difference between two parallel trackers (i.e. above which the exposed tracker would bear full wind loads).

There are infinite possible solutions for the combinations of the above variables, but only one optimal solution. Resolving this optimization problem is computationally intense, and manual solutions would be impractical due to extreme amounts of time required. What is needed is a set of algorithms to optimize the solution that can be run on a computer.

SUMMARY OF INVENTION

The subject of this invention is embodied in a cost-optimization device for the layout and construction of a utility-scale photovoltaic (“PV”) power plant. The optimization device employs a set of algorithms designed to find the most cost-effective solution under given conditions. The algorithms are written in computer machine readable code and are highly customizable for the specific tracker equipment requirements and owner/builder/maintainer specifications or preferences.

The preferred optimization device comprises three principal stages (or “units”) of computing: (1) an objective-state unit, (2) an optimum-feasible unit, and (3) a grading unit. In the first stage, the objective-state unit cost-optimizes site grading by orienting a ruling line between a maximum and a minimum pile reveal length for each tracker in the project (the objective-state solution”). When compared to the existing site topography, the ruling line indicates cost-optimized cut and fill locations.

In the second stage, the optimum-feasible unit modifies the objective-state solution to satisfy given geometric constraints of the project. More specifically, the optimum-feasible unit employs a project-wide mesh (“system mesh”) to check whether the objective-state solution complies with the project's geometric restrictions. If not, the optimum-feasible unit applies a marching algorithm to incrementally adjust each ruling line until it finds a solution that minimizes site grading while complying with the project's geometric restrictions (the “optimum-feasible solution”).

In the third stage, the grading unit modifies the optimum-feasible solution to satisfy non-geometric constraints of the project (the “final-state solution”).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a typical PV field with single axis trackers.

FIG. 2 illustrates a block diagram of the functional units of the optimization device 10.

FIG. 3A illustrates a method for carrying out the optimization in a flowchart.

FIG. 3B illustrates a hardware description of the optimization device 10.

FIG. 4 illustrates a sample correlation graph between energy yield and grading intensity.

FIG. 5 illustrates the least square ruling line adjustment for tracker piles.

FIG. 6 illustrates a ruled surface built by grouping of independent ruling lines.

FIG. 7 illustrates a definition of grading lines and tracker axis orientation parallel to the ruling line.

FIG. 8 illustrates topology of the system mesh 36.

FIG. 9 illustrates the iterative marching algorithm 44 with barriers.

FIG. 10 illustrates a way to monitor convergence.

FIG. 11 illustrates an example of an iterative marching algorithm.

FIG. 12 illustrates a typical ruled surface grading and piles example (from optimization device 10).

FIG. 13 illustrates piles' parametric model schematic.

FIG. 14 illustrates the nodes of a triangle in the original DTM are pushed up or down to find their new topographical elevations, per bi-linear interpolation of the adjacent ruler lines.

FIGS. 15 and 15A illustrate original terrain mesh.

FIGS. 16 and 16A illustrate proposed grading by reassignment of elevations for mesh nodes (optimized system mesh lines also depicted).

FIGS. 17 and 17A illustrate a tie to existing slopes for edge mesh triangles.

FIG. 18 illustrates node-i in the system mesh is manually pulled up to alter the system mesh locally.

FIGS. 19 and 19A illustrate an initial grading solution and selected system mesh link shown (fill and cut lines depicted).

FIGS. 20 and 20A illustrate a manual adjustment of the system mesh node, pulled upwards.

FIGS. 21 and 21A illustrate a resulting grading after manual adjustment of the system mesh.

FIG. 22 illustrates a hyperbolic paraboloid.

FIGS. 23 and 23A illustrate a grading pattern of a single block as a hyperbolic paraboloid.

FIGS. 24 and 24A illustrate the same blocks from FIGS. 23 and 23A graded as a plane. Note the better adaptability to the terrain in the first case.

FIG. 25 illustrates node “i” with a topographical elevation lower than all of its neighbors (a “low point”).

FIG. 26 illustrates a low point created by a cut line in its lower end and at an edge of the system mesh.

FIG. 27 illustrates a potential low end point corrected through tangency algorithm.

FIG. 28 illustrates and example of flow direction calculation with undisturbed terrain.

FIG. 29 illustrates an example of flow direction calculation after proposed grading.

FIG. 30 illustrates a zoom-in of flow direction inspection on an area with proposed terrain alteration.

FIG. 31 illustrates a typical depiction of contour lines.

FIG. 32 illustrates a typical depiction of color-mapped topographical elevations before grading.

FIG. 33 illustrates a typical depiction of color-mapped topographical elevations after grading.

FIG. 34 illustrates a triangle facet normal vector and projections to determine the triangle slope with respect to E-W and N-S directions.

FIG. 35 illustrates a typical depiction of highlighted triangles having an E-W slope larger than a certain threshold.

FIG. 36 illustrates a typical representation of pile reveal length distribution. Note the color scale legend in the top-right corner of the image.

FIG. 37 illustrates a typical list of piles with design and installation details.

FIG. 38 illustrates a typical representation of Tracker's N-S tilt angle distribution. Note the color scale legend in the top-right corner of the image.

FIG. 39 illustrates a solar system geometry as obtained with Optimization device 10 once imported in PVSYSt for solar system energy yield evaluation.

OVERVIEW OF THE PREFERRED EMBODIMENT

A broad overview of the preferred optimization device 10 is shown in FIG. 2. The preferred device comprises three basic computing units to arrive at a final-state solution 80. First, an objective-state unit 20 calculates an objective-state solution. Next, after comparing the objective-state solution against the project's geometric constraints, an optimum-feasible unit 40 modifies the objective-state solution (if needed) to reach an optimum-feasible solution that satisfies the project's geometric constraints. Finally, a grading unit 60 modifies the optimum-feasible solution to satisfy any non-geometric project-specific constraints to arrive at the final-state solution 80.

Next, the flowchart of FIG. 3A illustrates the preferred method for carrying out the invention. As shown, the first major step is performed by the objective state unit 20. The goal of the objective-state unit 20 is to calculate the objective-state solution 30. To accomplish this goal, the objective-state unit 20 should be configured to accept the following user input: geometric constraints 22, terrain data 24, and solar system layout 26. Using this input, the objective-state unit 20 can calculate the objective-state solution 30 as discussed in detail below. In addition, this input can be used to build a system mesh 36.

The next major step is performed by the optimum-feasible unit 40. The goal of the optimum-feasible unit 40 is to calculate the optimum-feasible solution state 58. To accomplish this goal, the optimum-feasible unit 40 begins with the objective-state solution 30. If the objective-state solution 30 is not a feasible solution (that is, the geometric restrictions fail), then the optimum-feasible unit 40 applies the marching algorithm 44 to modify the system mesh 36 an incremental amount toward a feasible solution 58. The optimum-feasible unit 40 repeats the marching algorithm 44 until a solution is found that both minimizes site grading and complies with the project's geometric restrictions 22 (the optimum feasible solution 58).

Once the optimum feasible solution 58 has been achieved, the grading unit 60 can modify the system mesh 36 until the final state solution 80 is found. The goal of the grading unit 60 is to translate the optimum feasible state system mesh 36 to a buildable grading design through alteration of the Digital Terrain Model 24 and pile design, per the basic grading algorithm 62. This process may require further changes in the optimum feasible solution 58 to satisfy other project-specific related constraints. FIG. 3A outlines the basic steps of the grading unit 60.

Next, a hardware description of the preferred optimization device 10 according to exemplary embodiments is described with reference to FIG. 3B. In FIG. 3B, the optimization device 10 includes a CPU 100 which performs the processes described below. A CPU 100 provides the processor for the objective-state unit 20, the optimum-feasible unit 40, and the grading unit 60 shown in FIG. 2. The CPU 100 includes a memory 110 in which process data and software instructions are stored. The data and instructions may also be stored on a separate storage medium disk such as a hard drive (HDD) or portable storage medium or may be stored remotely. The invention is not limited by the form of computer readable media on which the instructions are stored. For example, the instructions may be stored on CDs, DVDs, in FLASH memory, RAM, ROM, PROM, EPROM, EEPROM, hard disk or any other optimization device with which the CPU 100 can communicate, such as a server or computer.

Further, the claimed processing may be provided as a utility application, background daemon, or component of an operating system, or combination thereof, executing in conjunction with CPU 100 and an operating system such as Microsoft Windows 10, UNIX, Solaris, LINUX, Apple MAC-OS and other systems known to those skilled in the art. The hardware elements used for the information processing functions may be realized by various processing circuitry elements, known to those skilled in the art. For example, CPU 100 may be a Core processor from Intel or a processor from AMD, or may be other processor types that would be recognized by one of ordinary skill in the art. Alternatively, the CPU 100 may be implemented on an FPGA, ASIC, PLD or using discrete logic circuits, as one of ordinary skill in the art would recognize. Further, CPU 100 may be implemented as multiple processors cooperatively working in parallel to perform the instructions of the process described above.

The software instructions or algorithms for carrying out the process of cost optimization in accordance with this invention are described in detail below. The instructions are preferably in the form of software stored in the memory 110 that control the CPU 100 to perform the functions of calculating an objective-state solution 30, an optimum feasible solution 58, including a grading plan for minimizing earthwork under a plurality of tracker piles, and calculating a final-state solution 80, which is an adjusted objective-state solution that accounts for project-specific limitations. The hardware description above, exemplified by the structure example shown in FIG. 2, constitutes structure that is programmed or configured to perform the algorithms described below, which may be completely performed by the circuitry included in the single device shown in FIGS. 2 and 3B.

Description of Preferred Computing Units and Algorithms Objective-State Unit 20

1.1 Inputting Project Conditions and Restrictions

Minimizing site grading is key to controlling solar system project cost. Grading is often needed to adapt the terrain irregularities to the geometric restrictions and mechanical tolerances of the trackers. Minimum grading design seeks to limit site topography alterations while still complying with a project's geometric tracker restrictions 22 for a given solar system layout 26.

Turning to FIG. 3A, the optimization device 10 begins with data input of geometric restrictions (and mechanical tolerances) 22 of the single-axis trackers, a digital terrain model (DTM) 24 of the project site, and the solar system layout 26.

Site topography can be analyzed per two different length scales. Small scale terrain irregularities (or ‘terrain roughness’) are those with a characteristic length smaller than the tracker length. Large scale terrain irregularities (or ‘terrain orography’) are topographic irregularities with a characteristic length scale larger than the tracker length. In general, terrain roughness irregularities can be absorbed by the difference between the imposed maximum and minimum tracker torque tube height (TTH); i.e., the variation range the reveal length of the tracker foundation piles are given by design. This interval is defined as delta (δ) (see FIG. 7). δ=

TTH

_max−

TTH

_min

If for a certain tracker the terrain roughness scale is smaller than (δ), no grading would be needed under that tracker to locally smooth the terrain roughness. In practice, TTHmin is limited by the minimum clearance between ground and module frame when the tracker is fully deployed, and TTHmax is limited for project constructability reasons.

More relevant in evaluating the grading intensity is the large-scale terrain orography. The limits for which a certain terrain orography requires grading are given by the angular limitations the trackers are subjected to. These are (i) the maximum North-South tilt angle the torque tube is allowed, and (ii) the maximum East-West drop angle between two adjacent trackers. The first restriction accounts for the fact that North-tilted trackers yield less energy than South-tilted ones because of less-optimal exposure to solar beams (in the northern hemisphere). The second restriction accounts for the fact that excessive difference in the vertical elevation of two adjacent trackers would expose the higher tracker to full wind loads, thus increasing the structural costs. It also increases the mutual shading between solar modules mounted on adjacent trackers during operation, thus also impacting energy yield.

A third geometric control parameter is the maximum transversal slope of the East-West corridors between tracker alignments. In some instances, these corridors are designed as internal roads for construction and O&M purposes. The slope limitations for the transversal section of these corridors have an essential role in grading design as they entail the conditions for geometric continuity of the full system.

In summary, the geometric restrictions 22 defining the grading design parameters are the following:

-   -   Delta (δ) parameter, from foundation piles.     -   North-South maximum tracker torque tube tilt angle (α).     -   East-West maximum drop angle between adjacent trackers (β).     -   Transversal slope limitations for East-West corridors between         tracker alignments (γ).

For any given topography 24 and tracker layout 26, there are infinite grading solutions which satisfy a given set of geometric restrictions 22. A trivial solution would grade the full site as a single horizontal plane. By creating and leveraging “ruled surfaces”, grading intensity can be greatly minimized. Finding the minimum grading solution for a given set of restrictions is a computationally intense process which can only be resolved by means of numerical methods and 3D simulation techniques.

Alternatively, releasing the system restrictions and allowing for longer tracker foundation piles can also facilitate the solar system geometric adaptation to the existing terrain, thus reducing grading intensity. The trade-offs are reducing energy production and increasing steel foundation costs. When analyzing the most optimal design for a solar system, the Levelized Cost of Energy (LCOE) financial function is evaluated for its minimum value. This function is defined as the ratio between the life-cycle system cost and the corresponding energy yield, which are both function of the selected system geometric restrictions. Moreover, for each solar system there is a direct correlation between energy yield and grading intensity. Finding this correlation for is instrumental to find the most financially optimal design for the system. A sample correlation graph between energy yield and grading intensity is shown in FIG. 4.

1.2 Calculating the Objective-State Solution 30

Turning to FIG. 5, the minimum amount of earthwork under a tracker is found when the terrain 202 is graded to a theoretical straight line obtained by least squares adjustment to the points where the tracker piles 203 intersect the existing terrain 202. These are termed “ruling lines” 204. FIG. 5 illustrates the piles' alignment belonging to one tracker, the corresponding pile-terrain intersection points, and the ruling line 204 obtained through least-square fitting to these points. The ruling line for a tracker is contained in the vertical plane defined by the corresponding tracker's piles 203. This vertical plane is generally oriented in North-South direction, and also contains the tracker rotating axle (or torque tube 210).

As the terrain under each tracker has different orientations, the slopes and elevations of the several ruling lines 204 vary from tracker to tracker. When grouping a series of ruling lines, the geometric figure thus obtained is a three-dimensional ruled surface 206, as shown in FIG. 6.

A group of aligned trackers is called a “block” 212. This is defined as a parallelogram containing all the aligned trackers which ruling lines can form a ruled surface 206. Blocks 212 are considered grading units, so that grading parameters, geotechnical properties, pile geometry restrictions and all other design parameters are assigned to be identical for all the trackers within a block 212. Each ruler line 204 intersects the boundary of the block it belongs to at two points 208, located at its North and South ends respectively (see FIG. 6). By altering the elevation of these two endpoints 208, the ruler line slope and vertical position would be changed and thus the corresponding grading departing from optimal.

A relevant parameter to control grading intensity is given by the difference between the minimum and maximum reveal length allowed for the piles. This difference is called the “delta” 214, which limits the maximum scale of the terrain irregularities that the reveal lengths of the piles are able to absorb before requiring local grading. This “delta” value can be superimposed to the ruling line for every tracker by means of two parallel lines, called the “fill” (lower) and the “cut” (upper) lines (216, 218), as shown in FIG. 7. The minimum grading strategy which satisfies the “delta” condition for the piles then becomes cutting the terrain over the upper line (space below “cut” in FIG. 7) and filling the voids under the lower line (space above “fill” in FIG. 7). The tracker rotating axis 210 should be kept parallel to the ruling line 204.

By performing a site grading per the above procedure the minimum grading volume for a solar photovoltaic plant which is also compatible with the “delta” value for the piles would be achieved. Once the grading design is completed, the tracker rotating axis elevation is readjusted and the final reveal length of each of the piles determined. By changing the “delta” requirement for the piles both the pile lengths and grading intensity vary inversely, thus the combined cost can be determined and plotted against the corresponding “delta”.

The cost vs. delta graph can be used to find its minimum as the optimal design point for the Block. This minimum cost solution is essential in the full design optimization process, and is defined as the “Objective State” 30. Any deviation from this Objective State 30 implies additional grading volumes for a given “delta”.

1.3—Building the System Mesh 36

The objective-state solution 30 described above doesn't account for additional geometric restrictions 22 which are relevant for practical design. These are:

-   -   Maximum North facing tilt angle for tracker rotating axis:         Imposed by tracker manufacturer, limit energy yield loss and/or         surface water flow velocity limitations (scour).     -   Maximum South facing tilt angle for tracker rotating axis:         Imposed by tracker manufacturer and/or surface water flow         velocity limitations (scour).     -   Maximum East-West elevation difference between adjacent         trackers: Imposed by tracker manufacturer based on shielding         limitation for wind loads, energy yield loss due to mutual         shading and/or surface water flow velocity limitations (scour).     -   Maximum slope of the terrain between blocks: Imposed by grading         requirements for internal O&M roads and/or surface water flow         velocity limitations (scour).

In order to implement these restrictions, it is convenient to describe the geometry of the full photovoltaic system as a set of nodes 220 in the space, located at the end-points of the ruling lines. A series of virtual elements are defined connecting pairs of nodes. These elements are called “links”. Each ruler line is a link by itself, but additional ones are defined as needed to capture the full set of restrictions. For instance, connecting nodes belonging two adjacent Blocks to control the slope of the terrain between them. A graphical representation of the nodes and links (system mesh topology 36) is shown in FIG. 8.

Each node in the system is defined by an ordinal and the link connecting a pair of them (N_(i) and N_(j)) is identified with both subscripts (L_(ij)). The relevant variable at each node is its topographical elevation, so that node N_(i) has an elevation value of z_(i). The relevant variable for each link is its slope (α_(ij)), which can be quantified as the difference in elevation of its assigned nodes divided by the link's length:

$\alpha_{ij} = \frac{z_{i} - z_{j}}{L_{ij}}$

The system mesh topology 36 is fully defined when the elevation of each node in the system mesh is set. Assuming that the system has N-nodes, it is convenient to describe the “state” of the system as an N-dimensional arrays which coordinates are the topographical elevations of the sorted nodes: s ^(t)=(z ₁ ,z ₂ ,z ₃ , . . . z _(i) , . . . z _(j) , . . . z _(N))

Similarly, the restrictions can be written for the links as an M-dimensional array b, being M the total number of slope restrictions. The components of b consist of the slope limitations for the affected link times its length: b _(p)=α_(p) L _(p) where α_(p) is the maximum slope allowed for the link p. Each link may have more than one restriction, as is the case of the ruling lines which are constrained by both the North and the South facing slopes. It is possible to rewrite the set of restrictions as a M×N system of inequalities As≤b, where A represents the connectivity matrix between links and nodes. This system of inequalities is the mathematical formulation for the system constraints.

The “Objective-state solution” 38 obtained through the basic algorithm is just one point in the N-dimensional space. This point is called s^(obj), which is the minimum grading cost for the system but it doesn't in general fulfill all of the slope restrictions for its links. It is therefore an unfeasible solution for the optimization problem. This can assertion be written as: As ^(obj)

b In the N-dimensional space, the square of the distance between a feasible state s and the objective state s^(obj) is evaluated with the function:

${f(s)} = {{{s - s^{obj}}}^{2} = {\sum\limits_{i = 1}^{N}\left( {z_{i} - z_{i}^{obj}} \right)^{2}}}$

The Optimal Feasible Solution 58 is the set of node elevations s which minimize ƒ(s) and simultaneously satisfies the set of restrictions As≤b. Because the function ƒ(s) is quadratic, this optimization problem falls within the classical Constrained Quadratic Optimization category. Finding the exact solution to this complex problem is extremely costly in terms of both computer RAM requirements and time. Optimization device 10 employs a specific iterative marching algorithm which is not proven to provide the exact optimal solution (in mathematical terms), but still yields a sufficiently accurate optimal engineering solution.

Optimum-Feasible Unit 40

1.4 Applying an Iterative Marching Algorithm 44

The proposed iterative marching algorithm 44 starts with an initial feasible state s^(ini). The next state in the iteration is obtained by applying the Newton's method to seek the minimum value of the function ƒ(s), which measures the distance between the current state and the objective state s^(obj). This yields the following result:

$s_{k + 1} = {s_{k} + {ɛ\frac{\left( {s^{obj} - s_{k}} \right)}{{s^{obj} - s_{k}}}}}$ where (k) is the iteration step and where the marching parameter (ε) is a sufficiently small number. The following state (k+1) is obtained by displacing the previous one (k) in a small amount towards the direction of the objective-state solution. With the new state s_(k+1) computed, the slopes for all links are calculated to verify if all of them satisfy the system constrains: As _(k+1) ≤b

In affirmative case, a new state (k+2) will be computed per the same rule. The new state will invariably be closer to the objective state and therefore a better solution than the previous iteration. If the new state s_(k+1) doesn't fulfill the set of constraints, the nodes of those links violating their maximum slope will be incremented in the same amount in future steps, so to keep the slope of that link unchanged in further iterations.

The process can be explained graphically in the N-dimensional space. FIG. 9 shows a 2-dimensional space consisting of the topographic elevation coordinates of two nodes z₁ and z₂. The objective state 38 is defined by the point s^(obj) in the graph.

The initial state s^(ini) is located within the Feasible states region. The (unknown) feasible optimal solution is denoted by s* (38), and is the one state within the feasible region which is closer to the objective state.

Per this iterative algorithm, if the actual state s_(k) is in the feasible region, the new state s_(k+1) is determined by approaching one step forward towards the objective state through a straight line connecting the initial and the objective states. As soon as one or more constrains are violated, meaning that the feasible region limit has been reached at a barrier state, a new marching path is automatically set for the following iteration step. The new path is determined by maintaining the slope of the affected link at its limit, therefore the new path necessarily overlaps the feasible region border, and the next iteration step will yield a new state closer to s* and contained in the border of the feasibility region. s ^(obj)=(z ₁ ^(obj) ,z ₂ ^(obj))

While it is not guaranteed that this marching algorithm will hit the feasible optimal solution s*, it is straight forward to monitor the value of the function ƒ(s) as the marching solution progresses and determine if/when convergence is achieved. See, FIG. 9.

To define the initial feasible state s_(ini) a trivial option is selected by assuming the same value for the topographic elevation for all nodes, that is z₁=z₂=z₃= . . . =z_(N)=z_(min). If all nodes lie on the same plane, the slope of all the links would be zero, which guarantees that As^(ini)≤b. In practice, z_(min) is selected as the minimum elevation of all nodes in the objective state. For the marching parameter (ε) a maximum value of 1.2 inches is selected. This is more restrictive than standard grading accuracy or tolerances during construction of the plant.

FIG. 11 shows how the marching iterative algorithm works for a simple case. In the example in FIG. 11, the distance between a node in the objective state and its corresponding node in the initial state is divided into equal marching steps, being the maximum step across all nodes smaller than 1.2 inches. The marching steps are different for each node to match the same number of divisions (four in this example).

In the first iteration k=1, all initial state nodes are pulled up in one corresponding step and then slopes for links verified for their respective limits. After this first iteration, the new state is still feasible.

The same process is repeated in iteration k=2, but after the marching steps are applied the slope of link L₂₃ is found larger than its maximum limit. To avoid the slope of link L₂₃ growing further in future iterations, the marching step for node N₂ is set equal to the marching step of node N₃ in order to keep the slope of L₂₃ constant from k=1 onward.

After iteration k=3, the slope of link L₃₄ becomes larger than its limit, therefore the marching step for node N₄ is set equal to the marching step of node N₃ for further iterations.

After iteration k=4 the marching algorithm is completed with final position depicted in solid line.

Per this implementation, the optimum-feasible state could result in links with slopes in excess of its limits. However, by limiting the maximum step size to 1.2 inches the optimum-feasible state is well within the construction tolerances and the solution deemed acceptable for engineering purposes.

As a refinement, the optimum-feasible state can be slightly improved by shifting all nodes upwards or downwards in the same amount so to further reduce the value of the function ƒ(s). This additional general shift is found by least square techniques with respect to the objective state, which yields the following correction:

$\theta = \frac{{\underset{N}{\Sigma}z_{i}^{obj}} - {\underset{N}{\Sigma}z_{i}^{final}}}{N}$

The node elevations from the optimized state are transferred to the ruling lines and the terrain grading design completed for the applied “delta”. Details on the grading process and associated functions are explained in paragraph 1.5.

Grading Unit 60

1.5—Basic Grading Algorithm 62

Once the system mesh 36 has been iterated towards the feasible optimal solution 58 (s*), the existing digital terrain model (DTM) 24 is modified to the new proposed geometry (proposed grading solution). This is achieved by correcting the topographical elevations of the nodes in the triangular mesh of the original DTM 24. Because the (x,y) coordinates of the DTM nodes doesn't in general align with the system mesh elements, an interpolation algorithm is needed. A bi-linear (two dimensional) interpolation method is selected, so that a node which falls in between two consecutive ruler lines adopts a new elevation obtained through linear interpolation of the points belonging to the ruler lines and with the same y-coordinate. This is represented in FIGS. 14-16.

If the topographical elevation of the node in the existing terrain mesh falls within the “fill” and “cut” lines (216, 218), no change in elevation is imposed to that node. Therefore, the larger the distance between the parallel “cut” and “fill” lines (i.e., the “delta”) the lower the number of nodes in the terrain mesh that would need to be modified, and thus the intensity of the proposed grading.

1.6—Parametric Model for Pile Sizing

Foundations for tracker structures consist of steel piles directly driven into the terrain. In Optimization device 10, the reveal (exposed) length of the driven pile is limited by the maximum and minimum torque tube height design parameters for the Block. Both the steel section and embedded length of each pile depend on the loads and soil geotechnical characteristics. The soil-pile interaction has highly non-linear behavior and iterative calculations are needed for determination of stress level and deflections. These calculations are represented in the flow chart of FIG. 3A as calculate piles 66.

Instead of resolving the soil-pile interaction by means of the set of non-linear elastic foundation equations, Optimization device 10 uses a parametric model derived from a number of cases which were solved per these equations. Because the geometry and load range for piles supporting solar tracking structures has relatively small variation across manufacturers and project sites, it is possible to find a set of parametric equations describing the soil-pile interaction behavior with sufficient accuracy (+/−10% variation) and minimal computing cost. The set of equations has been obtained from least square adjustment of proposed curves based on data fitting. The parametric model equations solve for (i) the embedded length of the pile, (ii) the maximum bending moment, and (iii) axial and lateral-torsional buckling. The results of the parametric model for pile sizing purposes are the total length of the pile and the required steel section. FIG. 13 illustrates a schematic of a tracker foundation pile and variables in the parametric model.

The input data for the parametric model includes (i) reveal length of the pile, (ii) mechanical soil properties, (iii) soil corrosion level, and (iv) top-of-pile loads from tracker operation. Once the piles have been sized, the materials cost of steel can be determined for a given grading solution. Completing the cost equation for the project requires input of additional freight and installation costs data.

1.7—Slope Ties to Existing Terrain

As shown in FIG. 16, the modified triangles at the edges of the system boundaries have elevations different from the adjacent triangles in the undisturbed terrain mesh, therefore creating geometric discontinuity. This is resolved by calculating the tie slopes per a determined angle. FIG. 17 shows the result of the added mesh triangles tying the proposed graded terrain to existing terrain at system edges. This step occurs in the basic grading module 62 of FIG. 3A.

The algorithm to tie to existing terrain consists of the creation of additional triangles in the mesh. Each modified triangle at the edge of the system has one of its sides belonging to that edge (the “free edge”). Among the infinite number of planes containing a “free edge”, only one has the predetermined tie to existing slope. The intersection of that plane with the triangles out of the system boundaries define a series of points in the existing terrain. By connecting these points to the end points of the corresponding “free edge”, a series of new slope ties triangles are defined.

The envelope of the three dimensional region including the disturbed triangles and the new slope ties triangles is also extracted as a closed polyline in space for further processing of the resulting proposed surface in a separate Computer Assisted Drafting and Design (CADD) software.

1.8—Manual Manipulation of Grading Results 72

Per the Basic Grading Algorithm, the proposed alterations to the existing DTM are exclusively controlled by the geometry of the optimized system mesh 36, as obtained by the Optimization device 10 marching algorithm. In some instances, the user may need to alter the system mesh 36 manually to adapt the proposed grading results to design constrains other than the minimum grading target. Of course, any manual alteration of the system mesh will yield a grading solution departing from minimal earthwork.

To do so, the software preferably includes the ability to manually alter the elevation of the system mesh nodes and regrade accordingly per the basic grading algorithm. FIG. 18 illustrates the concept of manually pulling the nodes of the system mesh to alter the grading results locally.

This capability is implemented through the 3D graphic interface of Optimization device 10, by which the user can select a system mesh node on the screen and redefine its topographical elevation, thus distorting the system mesh locally. This action automatically re-grades the DTM to yield a different grading pattern in the affected area. FIGS. 19-21 show screenshots of this process.

1.9—Particular Cases of Ruled Surfaces

The geometry of a generic ruled surface can be further restricted to some particular cases:

-   -   Hyperbolic paraboloids: This surface is obtained when the swept         curves are also straight lines. See, FIG. 22, which illustrates         a hyperbolic paraboloid.     -   Planes: This surface is a particular case of hyperbolic         paraboloids when the swept straight lines are parallel.

While the terrain adaption provided by generic ruled surfaces yields the most cost-effective grading solution for solar plants equipped with single axis trackers, it may be convenient in some instances to restrict the geometry to hyperbolic paraboloids or even to planes. This is the case when the geometry and structural design of the single axis tracker requires the proposed grading surface to adapt to these restrictions. Optimization device 10 preferably has these options embedded in the algorithm to allow the user selecting these special cases when needed. To achieve these results, the system mesh constrains are activated accordingly within the marching algorithm. Examples are shown in FIGS. 23-24.

1.10—Grading Control Algorithms for Drainage Design

The Optimization device 10 marching algorithm for minimum earthwork design can be altered to predict and locally correct the grading pattern so to avoid a result that would otherwise impede the positive drainage of the disturbed terrain.

The algorithm for this process (included in 40) can be based on the fact that the optimized topology of the system mesh would create a water stagnation point (low point) if a node has a topographic elevation which is lower than all of its neighbor nodes. This potential situation is depicted in FIG. 25.

Before reaching this situation, a crosscheck process is preferably embedded in the marching algorithm so that the resulting elevation of each node is compared to the elevations of its neighbors at each step in the iteration. As soon as a “low point” is detected through the iterative process, the amplitude of the marching step for the affected node is set equal to the average of its neighbors, therefore eliminating this condition in the following iteration step. As the iterative process progresses, the location of the “low points” propagate through the system mesh until reaching the system edges, where theoretically positive natural drainage is achieved through the existing undisturbed terrain, out of the limits of the proposed grading.

This low point correction strategy inevitably distorts the minimum earthwork design point, in this case for the sake of project constructability.

It may occur that proposed grading still creates potential low points at the system boundaries. This is because of the difference in elevations between the system mesh lines and the corresponding “cut” and “fill” associated lines controlling the grading depth.

If a cut line produces grading at its lower end which happens to be located at the system edge, the proposed grading in that specific location is a potential low point. An example is illustrated in FIG. 26.

A similar situation would occur at the higher end of an active fill line.

To avoid these situations, the system mesh is automatically distorted at the system edges by setting the elevations of the cut (fill) end points to be the same as the existing terrain, where needed. The result of this corrective algorithm (included in the basic grading algorithm 63) is shown in FIG. 27.

A different situation that may impede positive drainage in proposed grading areas occurs if several adjacent system mesh links have small slope and similar elevations, thus creating an almost planar horizontal region, potentially prone to ponding. To avoid this possibility, the target objective solution can be automatically verified so that no links have a slope smaller than 2%. During the iterative marching algorithm 44, the system mesh approaches this objective solution, therefore reducing the possibility for final mesh links to have slopes smaller than this 2%.

Inspection of results and proceeding with manual adjustments where needed (as previously described in paragraph 1.7) is part of the engineering design process.

1.11—Tools for Inspection of Results

To facilitate the user's inspection of results and deciding if manual changes are needed to improve the solution locally, the following tools can be implemented by software:

Determination of Flow Directions

This tool can provides a 3D depiction of the run-off flow lines based on terrain elevations before and after grading. The algorithm is based on the calculation of upslope areas using a superimposed rectangular grid digital elevation model, with grid nodes' elevations adapted to the DTM triangular mesh. The procedure is based on representing flow direction as a single angle taken as the steepest downward slope from each rectangular cell upon inspection of the topographical elevations of its eight adjacent cells in the rectangular grid. The methodology follows the well-known recursive algorithm for single flow direction. FIGS. 28 and 29 shows an example of the typical output yield by this flow direction calculation method.

Once a grading solution is calculated, the detailed inspection of the run-off flow lines on the disturbed terrain helps the user to locate any potential low points and/or water stagnation areas. In these points, the manual regrading tool is used to find a local solution. See, FIG. 30.

Contour Lines

Depiction of terrain contour lines before and after grading helps the user to detect potential low points and/or flat horizontal areas. While the determination of flow direction provides a qualitative analysis (positive drainage test), the topographic contour lines visualization helps to quantitatively determine the slope of the terrain.

The algorithm for depiction of contour lines is based on the geometric fact that, if the line in space obtained as the intersection between the plane containing a DTM mesh triangle and a horizontal plane of given elevation intersects the DTM triangle sides, then this segment of the line within the DTM triangle is a contour line for the given horizontal plane elevation. By connecting all segments, the full map of contour lines can be found for the terrain for several horizontal planes at corresponding different elevations. An example of the contour line representation is shown in FIG. 31.

Terrain Elevation Heat-Maps

A different way to evaluate terrain elevation gradients and explore the DTM topography before and after proposed grading is by displaying each triangle in the DTM with a different color, the color being mapped per the topographical elevation of the triangle barycenter (z), where dark blue is lowest elevation in the DTM and dark red is the highest elevation. The equation for the Red-Green-Blue color combination (RGB scale) assigned to each triangle is as follows:

$p = \frac{z - z_{\min}}{z_{\max} - z_{\min}}$ if  p > 0.5 R = −8.0672p² + 13.701p − 00573 G = −3.9929p² + 4.019p + 0.019 B = 0 if  p <  = 0.5 R = 0 G = −3.9929p² + 4.019p + 0.019 B = −0.3221p² + 3.4062p + 0.6734 An example of the color-mapped topographical elevation graphic result is shown in FIGS. 32 and 33.

Terrain Slope Filters

The slope of each triangle in the DTM can be evaluated by calculating the normal vector of the plane containing the triangle. The vectors obtained by projections of the normal vector onto both the XZ and YZ planes are evaluated for their angle with respect to the horizontal plane XY.

FIG. 34 shows the normal vector (n) of a triangle facet and its corresponding projections onto the XZ and YZ planes (vectors n_(xz) and n_(yz), respectively). The angles between vectors n_(xz) and n_(yz), and the horizontal plane are α_(xz) and α_(yz), which determine the slope of the triangle facet with respect to the E-W and N-S directions respectively.

A graphic filter can be applied to all the triangles in the DTM so that triangles with N-S or E-W slopes higher than a specified threshold are highlighted in a different color. This provides a visual verification tool for the user to check that the design angular restrictions for the system mesh have been fulfilled through the marching algorithm process and correctly transmitted to the proposed grading.

An additional filter for the absolute slope of the normal vector can also be available for the user to verify areas with potentially high scour risk. See, FIG. 35.

Pile Heat-Maps and Pile Text File Output

Optimization device 10 can also provide a graphic representation tool to verify that the reveal length of the piles after proposed grading are within the input data limits. The difference between these two limits is the “delta” parameter, which has a relevant impact in the proposed solution and therefore in earthwork intensity.

The reveal length of the foundation steel piles after grading can be depicted as a color scale, the assigned color being mapped per the corresponding reveal length (dark blue for minimum reveal length, and dark red for maximum reveal length). Visual inspection of the pile reveal's distribution heat-map facilitates the validity of the proposed grading solution. An example of the graphic output is shown in FIG. 36.

Following the same principle, similar heat-maps are also available for graphic depiction of other geometric variables pertaining the calculated steel pile foundations, as the heat-maps for total length of the piles and for the embedded length of the piles. The former is useful for logistic purposes at the field during construction, as this representation provides geographic information for material deployment. The second is useful for construction schedule phasing and planning reasons, as pile driving time is proportional to embedded pile length.

Pile geometry data can compiled and listed on a pile-by-pile basis through an output text file. The tabulated information listed in the files includes: (i) the pile id number, block and sequential tracker numbers the pile belongs to, (iii) georeferenced X,Y coordinates for the piles, (iv) topographical elevations (georeferenced Z-coordinates) for ground elevation before and after grading at pile locations, top of pile elevation after installation, etc. (v) calculated steel section for the pile per the structural calculation parametric model. A sample of the pile data text file is shown in FIG. 37.

Single Axis Trackers' N-S Angle Heat-Maps and Text File Output

Optimization device 10 can also provide a graphic representation tool depicting the N-S tilt angle of the single axis trackers in the solar system. This helps the user to verify that the angular restrictions imposed to the trackers as input data set for the simulation are fulfilled after the proposed grading.

The N-S tilt angle of the trackers' torque tube (or axle) are shown per a color scale, the assigned color being mapped per the corresponding angle (dark blue for minimum negative values—oriented North—, and dark red for maximum positive angles—oriented South—).

The N-S tilt angle distribution of the trackers has a relevant impact in the energy production of the solar plant, as it affects the incidence angle of solar beams onto the solar photovoltaic modules mounted on the trackers. Optimization device 10 can automatically output a text file with the tracker identification number and the resulting N-S tilt angle. This tabulated data provides the user with sufficient detailed information to derive the frequency distribution of the N-S angle of the trackers, thus to estimate a geometric correction factor for the overall energy production of the solar plant with respect to a perfectly flat and horizontal solar plant layout.

A sample graphic output of the tracker's N-S angle distribution is shown in FIG. 38.

1.12—Interface with Other Engineering Software

Optimization device 10 preferably has built-in routines to export the simulation results to other industry-standard solar engineering software tools, commonly used to (i) estimate the energy production of the resulting solar system after the grading design has been optimized in Optimization device 10, and (ii) complete the construction exhibits (solar system engineering drawings) with other standard CADD tools. This is achieved by writing the geometric output of the simulations in the corresponding standardized interchange file formats.

Interface with Energy Yield Simulation Software

One of the industry standard software for energy yield simulations of solar photovoltaic systems is PVSyst © developed by the University of Genova, Switzerland. PVSyst allows a user to evaluate the energy production loss due to mutual shading of single axis trackers when the layout of the solar system is not contained in a single plane. To run this mutual shading analysis and determine its impact on the energy yield of the solar system, the user shall define the topographical elevations of each tracker. This can be done either manually or reading the so called “shading scene” from the specific file format compatible with the PVSyst software.

Optimization device 10 preferably can automatically produce the compatible “shading scene” files with the geometric data for all the trackers in the solar system for direct simulation of the energy yield estimations. FIG. 39 shows a sample “shading scene” obtained from a grading simulation in Optimization device 10, once rendered in PVSyst.

Interface with Computer Aided Drafting and Design (CADD) Software

Optimization device 10 preferably has a built-in function to export the DTM computational mesh in standard DXF file format. DXF is a file extension for a graphic image format typically used with CADD software. DXF stands for Drawing eXchange Format, which is widely used in the engineering industry to facilitate exchange of virtual models' entities and properties across different engineering software platforms.

With this Optimization device 10 functionality, the proposed grading geometry after simulation can be read by other CADD specific software where the final design engineering exhibits (civil construction drawings) are completed.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

The invention claimed is:
 1. A cost-optimization device for a utility-scale photovoltaic power plant, the cost-optimization device comprising: a central processing unit having a memory with instructions stored therein for causing the central processing unit to incorporate a set of geometric project restrictions and a set of non-geometric project restrictions to calculate a final-state solution that cost-optimizes site grading for the utility-scale photovoltaic power plant, wherein to calculate the final-state solution the central processing unit executes steps comprising: calculating an objective-state solution, wherein the objective-state solution comprises trackers, each tracker comprising a ruling line oriented to cost-optimize site grading, the ruling line located between a maximum and a minimum pile reveal length, calculating an optimum-feasible solution, the optimum feasible solution applying a marching algorithm to adjust each ruling line to accommodate the geometric project restrictions, the marching algorithm comprising: setting an initial feasible state for a first node that is within the geometric project restrictions, calculating a first node marching step by dividing a distance between an objective reasonable state of the first node and the initial feasible state of the first node by k iterations, determining a first iterative step of the first node by adding the first node marching step to the initial feasible state for the first node, setting an initial feasible state for a second node that is within the geometric project restrictions, calculating a second node marching step by dividing a distance between an objective reasonable state of the second node and the initial feasible state of the second node by k iterations, determining a first iterative step of the second node by adding the second node marching step to the initial feasible state for the second node, determining a slope between the first iterative step of the first node and the first iterative step of the second node, checking the slope against the geometric project restrictions, and if the slope is within the geometric project restrictions, then determine a second iterative step of the first node by adding the first node marching step to the first iterative step of the first node and determine a second iterative step of the second node by adding the second node marching step to the first iterative step of the second node, if the slope exceeds the geometric project restriction, setting the first node marching step and the second node marching step equal to each other for further iterations, and exporting the final-state solution as standardized interchange file formats by one or more of: (1) writing geometric output of the final-state to a standard solution shading scene file to be read by an energy yield simulation software application, and (2) writing geometric output of the final-state solution to a standard Computer Aided Drafting and Design (CADD) file shading scene file to be read by a CADD software application.
 2. The cost-optimization device of claim 1 further comprising, a system mesh, the system mesh comprising a plurality of nodes, blocks and links, each node comprising point on a ruling line, each block comprising a subset of ruling lines in the project, and each link connecting two nodes, wherein the optimum-feasible unit applies the marching algorithm to all links of the system mesh, including links between adjacent blocks. 